1. Field of the Invention
This invention generally relates to a method and apparatus for recovering carbon dioxide from a feed stream and reducing the costs of carbon dioxide purification systems. More particularly, the invention relates to a carbon dioxide purification method and apparatus utilizing an adsorbent bed, such as activated carbon, in combination with a catalytic oxidation system.
2. Description of the Related Art
Table 1 lists the concentrations of various components of a carbon dioxide feed stream from, for example, a well or an exhaust stream from a chemical process.
As shown, the feed stream contains various hydrocarbons that must be removed to provide relatively pure carbon dioxide. Currently, technologies such as scrubbers, adsorption systems and catalytic oxidation systems are employed to remove the hydrocarbons from the gas stream. These technologies are briefly discussed below.
Scrubbers generally utilize a water wash and are sufficient for removing water soluble hydrocarbons such as, for example, ethanol and methanol from feed streams. However, scrubbers are not effective for removing hydrocarbons that are not water soluble. Instead, adsorption beds and catalytic oxidation systems are generally used to remove non-water soluble hydrocarbons.
A typical adsorption bed includes activated carbon as an adsorbing medium. Such an adsorption bed is generally effective for inexpensively removing large quantities of hydrocarbons having boiling points greater than 20xc2x0 C. However, activated carbon beds are relatively inefficient in terms of capital and operating costs when the subject feed stream contains both weakly and strongly adsorbing species.(Basmadjian, p.75) For example, the weakly adsorbing species are typically displaced by the strongly adsorbing species and, consequently, not all hydrocarbon species are effectively adsorbed. Additionally, compounds such as ethane, propane, butane, methyl ether, etc., are not removed to the levels required (low ppm and/or ppb) for food grade carbon dioxide. For at least the foregoing reasons, an activated carbon system alone removes the contaminants only partially, for example only about 70% by weight of the hydrocarbon impurities in a feed stream described in Table 1 will be effectively removed by adsorption and therefore will not meet the specification of food grade carbon dioxide.(Table 5)
In addition to scrubbers and adsorption beds, catalytic oxidation systems may also be used to remove hydrocarbons from the feed stream. Catalytic oxidation systems are used for destroying volatile organic hydrocarbons and odorous compounds in exhaust air streams. Typically, for a feed stream containing ethane, propane and butane, catalytic oxidation over a platinum or palladium catalyst alone is effective even if water soluble and/or high boiling point hydrocarbon components are present. Since the feed gas may not contain adequate oxygen, oxygen must often be added to the feed stream to assure complete combustion of the hydrocarbons, and the amount and cost of the oxygen increases as the hydrocarbon level in the feed stream increases.
A significant amount of heat is generated when combusting high levels of hydrocarbons, and the heat must be removed to protect the catalyst and vessels. To limit heat generation, combustion is performed in multiple combustion stages. Heat generation in each stage may be controlled by limiting the amount of oxygen fed to each combustion stage, and by recycling carbon dioxide to reduce the concentration of hydrocarbons entering each combustion stage. Features such as multiple combustion stages, and oxygen limiting and heat removing systems, increase the complexity and costs associated with prior art catalytic oxidation systems.
By way of example, FIG. 1 illustrates a block flow diagram of a conventional three stage catalytic oxidation system for purifying the previously described feed stream. Table 2 contains an example of typical characteristics as the feed stream is being processed by the catalytic oxidation system depicted in FIG. 1.
Referring to FIG. 1, oxygen from a first oxygen source 30 (stream 17) is injected into a feed gas 10 stream 1) entering catalytic oxidation system 5 prior to the feed gas 10 entering a first heat exchanger 20. This oxygen provides an oxidant source for subsequent combustion of the feed gas 10 in a first reactor 40. The feed gas 10 is warmed in heat exchanger 20 as will be discussed below, enters the first reactor 40 (stream 2) and undergoes a catalytic oxidation process. The temperature of feed gas 10 in the first reactor 40 is measured by a first thermometer 50 and the amount of oxygen injected into the feed gas 10 by the first oxygen source 30 is controlled in accordance with the measured temperature. The temperature of the first reactor 40 is controlled to be about 875xc2x0 F. to ensure favorable reaction kinetics for combusting the hydrocarbons in the feed gas 10.
Feed gas 10 (stream 2) entering the first reactor 40 is brought up to the necessary activation temperature, about 500xc2x0 F., by passing through first heat exchanger 20. The first heat exchanger 20 uses a portion (stream 4) of the feed gas 10 exiting the first reactor 40 (stream 3) as a warming medium to warm the feed gas 10 entering the first reactor 40. The portion of the feed gas 10 used as the warming medium is then returned (stream 5) to join the remainder of the feed gas 10 (stream 6) exiting the first reactor 40.
As shown in Table 2, the feed gas 10 enters the first reactor 40 (stream 2), at 500xc2x0 F., with approximately 177.2 lbs. of hydrocarbons and a caloric value of 39.72 Btus/cubic foot of feed gas, and exits the first reactor 40 (stream 3) with approximately 121.93 lbs. of hydrocarbons and a caloric value of about 27.00 Btus/cubic foot of feed gas. Thus, approximately 29% by weight of the original hydrocarbons and about 32% of the caloric value are removed by the first catalytic combustion process. In this example, methane is not removed from the feed gas 10, but can be removed in a later processing operation in the carbon dioxide plant, such as in a stripper column where it is removed by distillation of the liquid carbon dioxide.
Following combustion in the first reactor 40, the feed gas 10 is successively fed to second and third reactors 80 and 120 (streams 7 and 13), respectively. More specifically, as shown in FIG. 1, the feed gas 10 receives oxygen (stream 18) from a second oxygen source 70 to provide an oxidant source for combustion in the second reactor 80. Prior to entry into the second reactor 80, the feed gas 10 is fed through a second heat exchanger 60 to raise the temperature of feed gas 10 (stream 10) to approximately 500xc2x0 F. (stream 8). The feed gas 10 then enters the second reactor 80 and undergoes a second catalytic combustion process. The amount of oxygen injected into the feed gas 10 prior to entering the second reactor 80 is determined in accordance with the temperature of the feed gas 10 in the second reactor 80 as measured by a second thermometer 90. Approximately 42% of the hydrocarbons and 53% of the caloric value in the feed gas 10 entering the second reactor 80 are removed by the second combustion process in this example.
A portion (stream 10) of the feed gas 10 exiting the second reactor 80 (stream 9) is used as a warming medium in the second heat exchanger 60 to warm the feed gas 10 flowing into second reactor 80. The feed gas 10 used as the warming medium is then rejoined (stream 11) with the remainder of the feed gas 10 exiting the second reactor 80 (stream 12).
Subsequent to exiting the second reactor 80, the feed gas 10 is fed (stream 13) to a third heat exchanger 100 in which the feed gas 10 is cooled to approximately 530xc2x0 F. The degree of cooling is determined in accordance with the temperature of the feed gas 10 in the third reactor as measured by a third thermometer 130. Of course, if necessary, heat may be added to the feed gas 10 in the third heat exchanger 100 to warm the feed gas 10 to approximately 530xc2x0 F. The feed gas 10 then passes into a third reactor 120 (stream 14). The feed gas 10 undergoes a third catalytic combustion process during which about 62% of the entering hydrocarbons and about 46% of the caloric value are removed. The oxygen content of the feed gas 10 exiting the third reactor 120 (stream 15) is monitored by an oxygen sensor 135 and an excess concentration of about 500-1000 ppm(v) is maintained by a third oxygen source 110 (stream 19).
The feed gas 10 exits the third reactor 120 (stream 15) and passes through a fourth heat exchanger 136 to cool the feed gas 10 to approximately 115xc2x0 F. The feed gas 10 is then fed (stream 16) to a carbon dioxide production facility (not shown).
The conventional multi-stage catalytic reactor system 5 discussed above and illustrated in FIG. 1 is effective for removing most hydrocarbons from a feed gas. For example, the catalytic reactor system discussed above removes about 84% of the hydrocarbons and about 84% of the caloric value present in an entering feed stream. However, such a system is relatively complicated and expensive to operate due to the relatively high caloric value of the feed stream.
A typical feed gas 10 from a chemical process may also include sulfur compounds in addition to the hydrocarbons previously discussed, and such sulfur compounds contaminate many conventional catalytic oxidation treatment facilities. The following U.S. patents illustrate technology used to remove volatile hydrocarbons from waste gas streams containing sulfur compounds.
U.S. Pat. No. 5,658,541 to Matros et al. describes a process and apparatus for removing volatile divalent sulfur compounds from waste gas streams. Volatile organic compounds are also converted to carbon dioxide and water vapor and are removed. Sulfur oxides resulting from a combustion process over a catalyst bed are removed by absorption or adsorption subsequent to the combustion process. The remaining waste gases, e.g., nitrogen, oxygen, carbon dioxide and water vapor, are vented to the atmosphere. During operation, sulfur salts build up on the catalyst bed and are periodically removed by raising the temperature of the catalyst bed to a reactivation temperature. The sulfur salts then decompose to form sulfur oxides and are purged from the catalyst bed. Further, Matros et al. provides for preheating the gas stream entering a combustion zone.
U.S. Pat. No. 5,061,464 to Cordonna et al. describes sulfur tolerant platinum group metal catalysts capable of oxidizing sulfur and carbon monoxide from a waste gas stream. U.S. Pat. No. 5,145,285 to Deeba et al. discloses a platinum on a titania or zirconia support. The disclosed catalyst may be used for the treatment of exhaust gases from vehicles and co-generation plants.
Although the Cordonna et al., Deeba et al. and Matros et al. patents provide processes for removing non-water soluble hydrocarbons from gas streams, these patents merely disclose catalytic oxidation systems producing waste gas streams which are vented to the atmosphere, and no attempt is made to remove the various compounds from the waste gas to provide a purified carbon dioxide product. In addition each of these systems described in these patents would require multistage catalytic reactors if the feed gas stream being treated has a high caloric value.
The prior art carbon dioxide systems discussed above generally do not provide an inexpensive and uncomplicated process and apparatus for removing hydrocarbons to low levels typical of stringent specifications for feeds that have a high caloric value. A need therefore exists for a more efficient carbon dioxide purification system for removing hydrocarbons from a feed gas with high caloric value. Such a system should preferably have the advantage of removing hydrocarbons inexpensively, and to a level permitted by, for example, stringent food grade specifications.
One aspect of the present invention is a process for purifying a feed gas which comprises predominantly carbon dioxide and further comprises hydrocarbon contaminants, the process comprising the steps of:
(i) adsorbing hydrocarbons from said feed gas to an extent corresponding to a sufficient reduction of the caloric content of the feed gas that the product stream produced in this step can be catalytically oxidized in a single catalytic oxidation reactor, wherein preferably said product gas has a sufficiently high caloric value that said catalytic oxidation can proceed autogenously; and
(ii) catalytically oxidizing hydrocarbons remaining in the gas stream produced in step (i).
Another aspect of the present invention is a carbon dioxide purification apparatus which includes (i) adsorption apparatus to adsorb hydrocarbons from a carbon dioxide feed gas onto an adsorbing material and produce a hydrocarbon-depleted gas stream, (ii) a catalytic oxidation reactor operatively connected to said adsorption apparatus to receive said hydrocarbon-depleted gas stream from said adsorption apparatus and oxidize residual hydrocarbons from said hydrocarbon-depleted gas stream, and (iii) apparatus for determining the caloric value of a hydrocarbon-depleted gas stream feed gas leaving said adsorption apparatus and diverting a portion of said gas stream around said adsorption apparatus as a function of said caloric value to provide that the gas stream produced in the adsorption apparatus can be catalytically oxidized in a single catalytic oxidation reactor, while providing in said gas a sufficiently high caloric value that said catalytic oxidation can proceed, preferably autogenously.
This invention will be particularly advantageous for carbon dioxide feed gases having a non-methane caloric value of greater than 12 Btus/standard cubic foot. This is due to the higher costs of the current option of a multistage catalytic oxidation system.
As used herein, xe2x80x9ccaloric valuexe2x80x9d is the heat (xe2x80x9cheat of combustionxe2x80x9d) produced by the complete combustion with oxygen of all the material, capable of such combustion, that is present in a gas stream, divided by the total volume of the gas stream. Heats of combustion can be found in published references such as the Chemical Engineers Handbook.